Microcavity plasma devices produce a nonequilibrium, low temperature plasma within, and essentially confined to, a cavity having a characteristic dimension d below approximately 500 μm. This new class of plasma devices exhibits several properties that differ substantially from those of conventional, macroscopic plasma sources. Because of their small physical dimensions, microcavity plasmas normally operate at gas (or vapor) pressures considerably higher than those accessible to macroscopic devices. For example, microplasma devices with a cylindrical microcavity having a diameter of 200-300 μm (or less) are capable of operation at rare gas (as well as N2 and other gases tested to date) pressures up to and beyond one atmosphere.
Work done by University of Illinois researchers is disclosed in U.S. Published Application Number 20070170866, to Eden, et al., which is entitled Arrays of Microcavity Plasma Devices with Dielectric Encapsulated Electrodes. That application discloses microcavity plasma devices and arrays with thin foil metal electrodes protected by metal oxide dielectric. The devices and arrays disclosed are based upon thin foils of metal that are available or can be produced in arbitrary lengths, such as on rolls. A method of manufacturing disclosed in the application discloses a first electrode pre-formed with microcavities having the desired cross-sectional geometry. Pre-formed screen-like metal foil, e.g. Al screens used in the battery industry, can be used with the disclosed methods. Oxide is subsequently grown on the foil, including on the inside walls of the microcavities (where plasma is to be produced), by wet electrochemical processing (anodization) of the foil. As disclosed in the application, providing a metal thin foil with microcavities includes either fabricating the cavities in metal foil by any of a variety of processes (laser ablation, chemical etching, etc.) or obtaining a metal thin foil with pre-fabricated microcavities from a supplier. A wide variety of microcavity shapes and cross-sectional geometries can be formed in metal foils according to the method disclosed in the application.
More recent work by University of Illinois researchers discloses buried circumferential electrode microcavity plasma device arrays and a self-patterned wet chemical etching formation method including controlled interconnections between. These results are disclosed in Eden et al., U.S. patent application Ser. No. 11/880,698, filed Jul. 24, 2007, entitled Buried Circumferential Electrode Microcavity Plasma Device Arrays, and Self-Patterned Formation Method, which has been published as WO 08/013,820 on Jan. 31, 2008 and as US 2008-0185579 on Aug. 7, 2008. In a disclosed method of formation in that application, a metal foil or film is obtained or formed with microcavities (such as through holes), and the foil or film is anodized to form metal oxide. One or more self-patterned metal electrodes are automatically formed and buried in the metal oxide created by the anodization process. The electrodes form in a closed circumference (a ring if the cavity shape is circular) around each microcavity, and can be electrically isolated or connected. Prior to processing, microcavities (such as through holes) of the desired shape are produced in a metal electrode (e.g., a foil or film). The electrode is subsequently anodized so as to convert virtually all of the electrode into a dielectric (normally an oxide). The anodization process and microcavity placement determines whether adjacent microcavities in an array are electrically connected or not.
Microcavity plasma devices fabricated in the metal/metal oxide structures described above are inexpensive, flexible and durable. Self-assembly processes can be used to automatically form the buried electrodes via anodization, as described above. However, prior microcavity plasma devices formed by semiconductor fabrication techniques in semiconductors and other materials have offered more control over the cross-sectional geometry (shape) of the microcavities than the anodization processes provided prior to the present invention. A tapered microcavity is provided in Eden, et al. U.S. Pat. No. 7,112,918, Sep. 26, 2006, which is entitled Microdischarge Devices and Arrays Having Tapered Microcavities. The tapered microcavity provides operational advantages, including improved extraction of light produced by plasma generated within the microcavity. However, the angle of the tapered sidewall of microcavities in silicon, for example, is fixed by the crystalline structure of the semiconductor.